1. Field of the Invention
This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to such a machine and method that constructs text or images from individual marks created on the printing medium, in a two-dimensional pixel array, by a pen or other marking element or head that scans across the medium bidirectionally.
The invention is particularly beneficial in printers that operate by the thermal-inkjet process--which discharges individual ink drops onto the printing medium. As will be seen, however, certain features of the invention are applicable to other scanning-head printing processes as well.
2. PRIOR ART
Bidirectional operation of any scanning-head device is advantageous in that no time is wasted in slewing or returning the print head across the medium to a starting position after each scan; however, bidirectional operation does present some obstacles to precise positioning of the printed marks, and also to best image quality. In order to describe these obstacles it will be helpful first to set forth some of the context in which these systems operate.
In many printing devices, position information is derived by automatic reading of graduations along a scale or so-called "encoder strip" (or sometimes "codestrip") that is extended across the medium. The graduations typically are in the form of opaque lines marked on a transparent plastic or glass strip, or in the form of solid opaque bars separated by apertures formed through a metal strip.
Such graduations typically are sensed electrooptically to generate an electrical waveform that may be characterized as a square wave, or more rigorously a trapezoidal wave. Electronic circuitry responds to each pulse in the wavetrain, signalling the pen-drive (or other marking-head-drive) mechanism at each pixel location--that is, each point where ink can be discharged to form a properly located picture element as part of the desired image.
These data are compared, or combined, with information about the desired image--triggering the pen or other marking head to produce a mark on the printing medium at each pixel location where a mark is desired. As will be understood, these operations are readily carried out for each of several different ink colors, for printing machines that are capable of printing in different colors.
In addition to this use of the encoder-derived signal as an absolute physical reference for firing the pens, the frequency of the wavetrain is ordinarily used to control the velocity of the pen carriage. Some systems also make other uses of the encoder signal--such as, for example, controlling carriage reversal, acceleration, mark quality, etc. in the end zones of the carriage travel, beyond the extent of the markable image region.
Now, standardized circuitry for responding to each pulse in the encoder-derived signal is most straightforwardly designed to recognize a common feature of each pulse. Thus some circuits may operate from a leading (rising) edge of a pulse, others from a trailing (falling) edge--but generally each circuit will respond only to one or the other, not both.
Such circuits have been developed to a highly refined stage, for use in printers that scan only unidirectionally. Accordingly it is cost-effective and otherwise desirable to employ one of these well-refined, already existing circuits in a machine that scans bidirectionally as well; however, in adapting such a preexisting design for use in a bidirectional machine, two and sometimes three problems arise.
(a) Encoder dimensional tolerances--FIG. 8 illustrates the situation, under the assumption (but only for definiteness) that the encoder-reading circuitry is triggered from falling edges 14 (in other words 14a, 14b, . . .) of the initial encoder-derived wavetrain 13. The alternating opaque markings 11 and transparent segments 12 (or solid bars and orifices) of the encoder strip 10 are shown in time alignment with the signals 13, 16 that result from reading of those features by a transmissive optical emitter/detector pair.
FIG. 8 shows that the falling edges 14, 17 do not occur at the same physical locations along the strip 10 during operation in opposite directions. (The drawing represents scanning forward by time values t.sub.F increasing toward the right, in one plot 19.sub.F of signal strength S.sub.F vs. time t.sub.F --and scanning backward by time values t.sub.B increasing toward the left in another, lower such plot 19.sub.B of S.sub.B vs. t.sub.B.) To put it another way, what constitutes a falling edge is different 14, 17 when the carriage moves in opposite directions.
Thus when the carriage moves from left to right, a falling edge 14 is at the right end of each positive square wave; but when the carriage moves from right to left the falling edge 17 is at the left end. These two positions are separated by the width T of a transparent segment (or orifice) 12 of the encoder strip 10.
It will be understood that, in selecting the point at which a mark should be made, it is possible to make allowance for the nominal width of the transparent segment 12. For example, the firing of a pen could be delayed by a period of time automatically calculated from the nominal width of the transparent segment 12 divided by the carriage velocity. Although both these pieces of information are available during operation of the system, the results of this method would be unsatisfactory because of preferred manufacturing procedures for creation of the encoder strip 10. These procedures arise from economics related to dimensional requirements, as follows.
In making the encoder strip 10, the dimension which is most important to hold to highest precision is the overall periodicity P of the alternating opaque bars 11 and transparent segments 12--i. e., the dimension P that gives rise to a full wavelength of the wavetrain. The two internal dimensions of each mark-and-transparent-segment pair--namely, the length B of the bar 11 and the length T of the transparent segment 12--are much less important, particularly if the encoder strip 10 is made for use in a machine that scans only unidirectionally.
In a unidirectional printing machine, only the distance between falling edges 14 (or rising edges 15) has any importance, provided only that (1) the distance B from each falling edge 14 to its next associated rising edge 15 is great enough to permit the sensing apparatus to recognize the falling edge; and (2) the distance T from each rising edge 15 to its next associated falling edge 14 is great enough to permit the sensing apparatus to reset itself in preparation for sensing the falling edge.
More specifically, the dimensional accuracy of the encoder-strip features, as shown in FIG. 8, are plus-or-minus only one percent for the full periodic pattern width P, but plus-or-minus ten to twenty percent for the opaque bar width B alone. If the bidirectional encoder signals 13, 16 are referred to opposite ends of an opaque area or bar 11, the relative accuracy of the positioning in opposite directions tracks the dimensional accuracy of the opaque area 11, namely plus-or-minus ten to twenty percent of nominal width B of the opaque bar.
It would be entirely possible to manufacture an encoder strip with much finer precision in the internal dimensions B, T just mentioned. An encoder strip so made, however, would be substantially more expensive.
Furthermore, it would be wasteful or at least uneconomic to use such an expensive strip in machines that scan only unidirectionally. On the other hand, it would be undesirably expensive to make and stock two different kinds of strip (one inexpensive one for undirectional machines; and another, more expensive, one for bidirectional machines).
Heretofore, accordingly, economical precise bidirectional printing has been deterred by a troublesome choice between two alternative problems: either bidirectional precision is poor, because of imprecisions in the internal dimensions B, T of the encoder-strip features 11, 12; or undesirable expense is incurred in providing high precision in these features.
(b) Time-of-flight and analogous misalignment effects--A certain amount of time elapses between the issuance of a mark-command pulse to a print head and the mark actually being created on the printing medium. For instance, in an inkier printer, some time elapses between:
the issuance of a fire-command pulse--approximately at an encoder-wavetrain falling edge 14a (FIG. 9)--to a pen 31 nozzle and PA1 the instant when a resulting ink drop 32 actually reaches the medium 33.
During this time, however, the carriage and pen 31 continue to move across the printing medium 33--and, in the case of an inkjet device, so does the ink drop 32, even after leaving the pen 31. The initial velocity component .about.v.sub.cF of the drop 32 along the scanning axis or dimension, when scanning forward, is very closely equal to the carriage velocity v.sub.cF ; this velocity likely decreases (though this is not illustrated) while the drop 32 travels in the orthogonal axis or dimension toward the printing medium 33--but nevertheless, as shown in FIG. 9, some forward movement or displacement .DELTA.x.sub.F of the ink drop 32 along the scanning axis does occur before the drop 32 reaches the medium 33 to form an ink spot 34.
In a printing machine that scans unidirectionally, this delay is substantially inconsequential, for all the ink drops 32 are offset in this same manner by very nearly the same distance, and in the same direction. In other words, the entire image is offset together along the scanning axis; but this does not matter to the resulting printed image because there are no relative offsets within the image--and therefore no discontinuities, no distortions of image features, etc.
As further shown in FIG. 9, however, during scanning in two opposite directions the respective offsets .DELTA.x.sub.F, .DELTA.x.sub.B that occur are likewise in opposite directions. The result is that, even if pen firing in opposite directions can be triggered at precisely the same point 14a, 18a along the encoder strip 10, the total mutual offset .DELTA.x.sub.T =.DELTA.x.sub.F +.DELTA.x.sub.B between two resulting image elements is approximately twice the value .DELTA.x.sub.F or .DELTA.x.sub.B of an individual time-of-flight-generated offset.
In consequence, when a swath of marks 34 is produced while the marking device 31 travels in one direction ("forward") F, and then another swath 35 is produced while the device 31 travels in the opposite direction ("backward") B, the features 34, 35 constructed in the two swaths will be mutually misaligned. The errors, in a word, are additive.
Physically speaking, the above-described relationships obtain in any prior-art bidirectional inkjet printer. The prior art, however, appears to provide neither recognition of these relationships nor measures to overcome the resulting misalignments.
These adverse effects are not necessarily limited to inkjet devices. Some slight marking delay within the electronic system (and mechanical system, when present) also occurs in other types of scanning printers--such as, for example, dot-matrix or even thermal-paper devices. In principle such delay perhaps can be reduced to a negligible magnitude in a system that is designed from the outset with bidirectional scanning in mind.
Adaptation of already existing unidirectional systems to bidirectional operation, however, may be uneconomic if relatively large marking delay happens to have been built into the original unidirectional system design at a relatively fundamental level. It will be understood that there may have been little motivation for avoiding such a relatively large delay in a unidirectional system, since such delay is readily and satisfactorily compensated at other points in the overall timing.
Thus time-of-flight and analogous misalignment effects impede the effective use of bidirectional printing for creating high-accuracy images. These effects are substantially independent of the imprecisions discussed in the preceding section.
(c) Image mottling--When inkjet printing systems are refined for high color saturation on transparency printing stock, it has been found desirable to put down two (or even more) drops of ink at each pixel location. This treatment provides high color saturation of primary and secondary colors, resulting in color images that are very appealing--and also expanding the gamut of complex colors that can be printed.
It has been noted, however, that when such systems operate bidirectionally, and when timing of the ink-drop firing is made very precise, the printed transparencies exhibit unacceptable "mottling" in solid color-filled areas--particularly for cyan. This visual effect is quite unpleasant and would decrease the value of the printing system to consumers.
One way to avoid this problem is to provide more effective drying, as for example by operating the printer more slowly to provide more drying time between pen passes over the transparency stock. Slower operation, however, unacceptably decreases overall throughput (e. g., pages per unit time) of the work.
U.S. Pat. No. 4,617,580 of Miyakawa teaches that low liquid absorption of transparency film can be combatted in liquid-ink printing by using a plurality of smaller ink droplets onto what would ordinarily be considered a single-pixel area--with the droplets being systematically shifted slightly from one another by a predetermined distance. U.S. Pat. No. 4,575,730 of Logan attempts to correct nonuniform appearance of large-area inkjet printing, referred to as "corduroy texture of washboard appearance", by overlapping of ink spots randomly. It has not been taught, however, how to apply such techniques both economically and effectively in bidirectional printing, particularly in the context of a preexisting machine architecture.
As can now be seen, important aspects of the technology which is used in the field of the invention are susceptible to useful refinement.